Method for Electrolytic Stripping of Spray Metal Coated Substrate

ABSTRACT

A method for stripping a fused thermal spray metal coating from the surface of a soft metallic substrate. The steps for the method include: immersing the coated metallic substrate in an aqueous solution of chromic acid, peroxide, and a second acid; immersing a metal cathode in the aqueous solution; applying a positive potential to the fused spray metal coated substrate and a negative potential to the metal cathode to generate a direct current between the substrate and the cathode; where the current is applied for a sufficient time to remove the coating. This method permits the electrochemical removal of fused and impermeable thermal spray metal coatings.

FIELD OF THE INVENTION

The present invention relates generally to a method forelectrochemically stripping a spray metal coating from an electricallyconductive material.

BACKGROUND OF THE INVENTION

Surface treatment of a substrate is known to control friction and wear,improve corrosion resistance, appearance, and/or alter the dimensions ofthe substrate. There are a variety of surface treatments available,including painting, laminating, physical vapor deposition and chemicalvapor deposition. In addition, two common surface treatments used onmetal substrates are electroplating and thermal spray coating (alsoknown as hardfacing).

Electroplating or plating is an electrochemical process in which a metalcoating is deposited on a substrate by passing a current through thesubstrate placed in an electrolytic bath containing metal ions.Electroplated coatings are widely used for depositing a corrosion- orwear-resistant metal on the substrate.

Thermal spraying is a coating process that consists of a heat source anda coating material feedstock. The coating material can often be in apowder- or wire-form, and is sprayed onto a substrate in a molten state.The thermal spraying process welds the coating material to thesubstrate. Thermal spraying methods using high velocity processes suchas flame spray, plasma, high velocity oxygen fuel (HVOF), electric-arc,and detonation gun (D-gun), are widely used.

The plasma spray, HVOF and detonation gun (D-gun) processes, usedifferent approaches for melting a metal powder, and propelling theresulting metal droplets onto the surface to be coated. They produce acoating layer made of consecutive layers of solidified metal droplets.The result is a rather porous coating with different degrees of coatingoxides included in the coating.

Flame spray, also known as oxy-acetylene combustion spray, uses thebasic principles of a welding torch to propel molten particles onto thesubstrate. The coating material can be either a wire- or powder-form.Usually nitrogen is used as a carrier to conduct metal powder into thecentre of the combustion zone in the torch.

Thermal spray metal coatings are naturally somewhat porous or permeable,since the molten metal droplets upon impact with the substrate don'tnecessarily merge into a continuous, impermeable coating. To achieve animpermeable coating, thermal spray metal coatings are often fused afterbeing applied to enhance bond strengths and coating density. The fusingprocess at least partially melts and bonds coating particles to eachother, resulting in a non-porous coating virtually free from coatingoxides found in non-fused spray coatings. This optional step changes theprocess from a “cold spray” method to a “spray-and-fuse” method. Coldspray coatings exhibit lower bond strengths than most other thermalspray processes. Spray-and-fuse coatings are used in applications whereexcessive wear and/or corrosion combined with high stresses on thecoating/substrate are a problem.

Methods for the removal of electroplated coatings essentially involve aprocess known as reverse electroplating, the electrical reversal of theplating process. Reverse electroplating involves applying an electriccurrent through an electrolytic bath, wherein the positive electricalpotential is applied to a coated metallic substrate and a negativeelectrical potential is applied to a cathode. A direct current isgenerated between the substrate and the cathode, removing the coating.Electroplated coatings can usually be completely removed without damageto the substrate, given sufficient care and attention.

The prior art describes electrochemical methods for removingelectroplated coatings. U.S. Pat. No. 4,356,069 (Cunningham) describes amethod wherein electroplated chrome and nickel coatings can be strippedusing an electrochemical process. The patent discloses a strippingcomposition which aids in the removal of chromium and nickel ions fromthe surface of a base metal, such as zinc, steel, aluminum, brass orcopper which had been previously plated with chrome and/or nickel“.

Electrochemical processes have generally been accepted in the art asunsuited for the removal of fused thermal spray coatings since thecoatings were considered insufficiently porous to be susceptible to theknown stripping compositions. In fact, Cunningham explicitly states thatthe process is for use on metals previously plated (column 2, line 35).The term “plated” is known in the art to refer to the deposition of acoating of metal using electrolysis. Cunningham does not mention thermalspray coatings in general, or flame spray-and-fuse coatings inparticular. Furthermore, the harsh stripping compositions generally usedwere expected to result in the corrosion of the metal substrate,especially iron substrates.

Up to this point, there has been no reason to expect that the fusedthermal spray coatings would be susceptible to stripping compositionattack due to their lack of porosity and that the extremely harshchemical stripping composition necessary for electrochemical removal offused coatings would not damage the underlying substrate if used forremoving fused thermal spray metal coatings from soft metal substrates.The prior art does describe electrochemical methods for removing thermalspray metal coatings, but not fused coatings, and the disclosed methodsare limited to situations where a person of skill in the art wouldexpect that the metal substrate would not be corroded by an acidicelectrolytic bath.

Thermal spray or electroplated coatings generally become worn duringuse, and the substrates are often of sufficient value that it isdesirable to reuse or recycle them. For reuse, it is necessary to stripthe coating, re-coat the substrate, and place it back in service. Thisrequires a means of economically and quickly stripping the coatingwithout damaging the base metal substrate of the object it is alsosometimes desirable to strip the coating and re-coat the substrate if,in the process of coating a part, the part no longer meetsspecifications. This, also, must be accomplished without damaging thesubstrate.

Traditionally, removal of thermal spray coatings from a substrate hasbeen accomplished using grinding or other mechanical means, such assand, shot, or grit blasting. However, parts with complex shape cannotbe ground within the desired tolerances, and the other mechanicalmethods are also not sufficiently precise. Moreover, the removal ofthermal spray coatings using mechanical means can result in irreparabledamage to the substrate.

It is frequently necessary or desirable to roughen the surface of asubstrate rather deeply before thermal spray metal coating in order toachieve the necessary bonding strength of the coating to the substrate.Therefore, coatings must be completely removed before the substrate isre-coated with thermal spray coatings. Since the thermal spray coatingcannot be completely removed by mechanical means without also removing aconsiderable amount of the substrate material, serious and oftenunacceptable changes to the dimensions of the substrate can occur.

U.S. Pat. No. 4,128,463 (Formanik) discloses a process wherein titaniumor titanium alloy substrates coated with flame-spray metal WC coatingsare stripped using an electrolytic cell. Titanium and titanium alloysare known in the art to be highly resistant to corrosion and would notbe expected to be damaged during the reverse electroplating process. Itis, therefore, not unexpected that reverse electroplating could be usedto remove a spray metal coating without damaging the titanium substrate.However, Formanik does not discuss or even suggest that the processcould be used for other substrates. The process disclosed by Formanik isonly described for use in removing WC coatings from a titanium or atitanium alloy, substrate, and no soft metal substrates such as iron orsteel. More importantly, Formanik never discusses fused spray metalcoatings and whether the disclosed process could be used for the removalof fused thermal spray metal coatings.

U.S. Pat. No. 4,886,588 (Curfman) discloses a process wherein spraymetal coatings can be electrochemically removed from a soft metalsubstrate, in this case aluminum. However, aluminum is not as corrosionresistant as titanium. Thus, it is not surprising that the focus of theprocess described by Curfman is to prevent corrosion of the substrate byusing aluminum corrosion inhibitors in the reverse electroplatingprocess. It is taught by Curfman that preventing the corrosion of themetal substrate by the highly corrosive electrolytic bath used forreverse electroplating is of great importance (see column 2, lines7-11).

Neither Formanik nor Curfman disclose a process for the removal of fusedthermal spray coatings or other coatings of low permeability (e.g. flamespray-and-fuse coatings, plasma spray coatings, or HVOF coatings). Infact, Formanik describes the process as mechanically loosening thecoatings by electrolytically generating hydrogen gas on the surface ofthe substrate (see column 1, lines 61-64). For this mechanical looseningto occur, the coating must be permeable. Formanik and Curfman neitherdisclose nor suggest the electrochemical removal of low permeability,fused coatings from steel, mild steel, common steel or iron.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate at leastone disadvantage of the references discussed above.

It has been surprisingly discovered that electrochemical removal offused thermal spray metal coatings is possible.

It is surprising that electrochemical conditions were discovered thatcompletely remove flame spray-and-fused coatings economically from asoft metal substrate, without altering the substrate.

In a first aspect, the present invention provides a method for strippinga fused spray metal coating from the surface of a soft metallicsubstrate. The steps for the process comprise: immersing the coatedmetallic substrate as an anode in an aqueous solution of chromic acid,peroxide, and sulphuric acid or phosphoric acid; immersing a metalcathode in the aqueous solution; applying a positive potential to thefused spray metal coated substrate and a negative potential to the metalcathode to generate a direct current between the substrate and thecathode; whereby the current is preferably applied for a sufficient timeto completely remove the coating.

In various embodiments, the spray metal coating is a NiCr, WC or MoS₂alloy. In various embodiments, the metallic substrate is steel, commonsteel or iron. In various embodiments, the aqueous solution is made upof: 6 to 38 parts by volume of a 20 to 25 wt % aqueous chromic acidsolution; 2 to 8 parts by volume of about a 35 wt % aqueous peroxidesolution; and 100 parts by volume of 55°-35° Baume sulphuric acid. Whileany peroxide is acceptable, the aqueous solution preferably compriseshydrogen peroxide or barium peroxide. In further embodiments, thechromic acid is H₂CrO₄, and the positive potential applied generates acurrent of 20 to 80 mA/cm² (milli Amperes per square centimeter) (20 to80 A/ft²).

In another aspect, the aqueous solution further includes a catalyst,which, in the embodiment above, is peroxide. In a further aspect, thetemperature of the aqueous solution is maintained at a temperaturebetween 30° C. and 80° C., (90° F. and 180° F.) and preferably between50° and 60° C. (120° F. and 140° F.).

In a further embodiment, the cathode is solid lead, carbon, titanium orany other conductor inert to the electrolyte solution. The cathodepreferably has a conductive material, the conductive material having anon-conductive backing and a coating of lead, carbon, titanium or otherconductor inert to the electrolyte solution which has beenelectrochemically deposited on the conductive material. Cathodes of thisembodiment are lighter and more easily maneuvered than solid metalcathodes.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention.

DETAILED DESCRIPTION

Generally, the present invention provides a method and system forelectrochemically removing fused spray metal coatings from metallicsubstrates. For the purpose of illustrating the preferred embodiment,there will be discussed a method for the use of a stripping composition,which is adapted for the electrochemical removal of chrome and/or nickelspray metal coatings. However, other alloys such as WC or MoS₂ can alsobe removed by this process.

The electrochemical stripping of a coating depends upon the object beingplaced in a bath containing chemicals such that one of the coatingcomponents is either soluble or will react with a chemical in the bathto form a soluble compound. Preferably, the reaction between thechemicals in the bath and the coating should be slow, otherwise theprocess would be difficult to control and there would be greater risk ofdamaging the substrate.

In some cases, the reaction will not proceed because compounds areformed that coat the surface and stop the reaction. In others, theprocess would be uneven because the reaction would be catalyzed byimpurities in places. The application of an electrical potentialovercomes these problems with the correct placement of the electrodes.

The application of an electrical potential enables a weak reaction to beaccelerated and controlled to prevent damage to the substrate. In somecases, corrosion inhibitors are helpful to protect the substrate.

A final advantage of electrochemical stripping is that the concentrationof the chemicals in the bath are maintained because the material beingremoved is redeposited at the other electrode and the chemical stripperis thereby rejuvenated. This also helps in the control of the processbecause the rate of stripping is more or less constant.

The stripping composition of the present invention is an aqueous,electrolyte solution having the following ingredients: chromic acid(CrO₃), a peroxide, preferably hydrogen peroxide (H₂O₂), a second acid,preferably sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄), and water(H₂O). The method of preparing the stripping composition is alsoimportant since exothermic reactions are involved and the order ofaddition of the ingredients is significant.

Thus, it is preferred that the chromic acid solution be initially formedhaving a 20-25% CrO₃ concentration by weight in water. In the presenceof water, CrO₃ forms H₂CrO₄. Preferably 225-1500 ml (6-40 parts byvolume) of this chromic acid solution is added to each gallon (100 partsby volume) of sulfuric acid. In terms of dry weight of CrO₃ added toeach gallon of H₂SO₄, this ranges from 2-10 ounces, and is preferably 24ounces. The sulfuric acid itself is an aqueous solution of H₂SO₄ andwater. Preferred is a 50° Baume solution, i.e. one having a 62.2% H₂SO₄concentration by weight and a specific gravity of 1.53. Other sulfuricacids in the 55°-35° Baume range may be used as well. Phosphoric acid atthis concentration may be used as well. The chromic acid solution isadded to the sulfuric acid preferably at between room temperature and50° C.

Because an elevated temperature may be used and since the reactionbetween CrO₃ and H₂SO₄ is exothermic, the solution should thereafter becooled. Once cooled, the peroxide is added. Preferred is a hydrogenperoxide which has a 35% H₂O₂ concentration by weight in water.Technical grades of hydrogen peroxide at this concentration areavailable. Other concentrations may also be used, as may otherperoxides, such as barium peroxide. The preferred amount is 100-300 ml(2.5-8 parts by volume) of this peroxide solution. Although more may beadded without deleterious effect, it is not necessary to achieve theresult desired.

The stripping solution is now complete in terms of active ingredients.Clearly, it may contain amounts of any number of inactive ingredients aswell. The exact chemistry of the products within the stripping solutionafter mixing is not known. In the preferred embodiment, however, it isbelieved that a trivalent form of chromium such as chromium sulfate orchromium dichromate is formed and that the trivalent chromium isoxidized at the anode to form a hexavalent chromium. In the process, thechromium metal plated on the base metal substrate of the anode will bestripped therefrom. The peroxide is believed in the process to form anintermediate oxide layer on the surface of the base metal substratewhich will protect the substrate metal from attack by the sulfuric acid.

A chromic acid solution is prepared having a CrO₃ concentration of 25%.20 parts per volume of this solution are gradually added to 50° Baumesulfuric acid at room temperature. The rate of addition is controlled toprevent overheating of the resulting composition. Alternatively, thecomposition is cooled during addition of the chromic acid solution. 4parts per volume of a 35% aqueous hydrogen peroxide solution are addedonce the composition is at room temperature to finish the strippingcomposition. A lead cathode is inserted into the stripping compositionand a steel PCP pump rotor with a fused thermal spray coating of NiCr issuspended in the stripping composition. A positive electrical potentialis applied to the coated rotor and a negative electrical potential isapplied to the lead cathode. A current is applied for 30 minutes forcomplete removal of the fused NiCr coating.

In the preceding description, for purposes of explanation, numerousdetails are set forth in order to provide a thorough understanding ofthe embodiments of the invention. However, it will be apparent to oneskilled in the art that these specific details are not required in orderto practice the invention.

The above-described embodiments of the invention are intended to beexamples only. Alterations, modifications and variations can be effectedto the particular embodiments by those of skill in the art withoutdeparting from the scope of the invention, which is defined solely bythe claims appended hereto.

1. A method for stripping a fused spray metal coating from the surfaceof an iron based substrate, comprising the steps of: immersing the fusedspray metal coated iron based substrate in an aqueous solutioncomprising chromic acid, peroxide, and a second acid, the second acidbeing sulfuric acid, phosphoric acid or a mixture thereof; immersing ametal cathode in the aqueous solution; and applying a positiveelectrical potential to the substrate and a negative electricalpotential to the cathode for creating a direct electrical currentbetween the cathode and the anode through the fused spray metal coatingfor removal of the fused spray metal coating from the steel substrate.2. The method according to claim 1, wherein the positive and negativeelectrical potentials are maintained until complete removal of the fusedspray metal coating.
 3. The method according to claim 1 or 2, whereinthe spray metal coating is a NiCr, a WC, or a MoS₂ alloy.
 4. The methodaccording to claim 1, 2 or 3, wherein the substrate is steel, mildsteel, common steel, or iron.
 5. The method according to any one ofclaims 1 to 4, wherein the spray metal coating is an impermeablecoating.
 6. The method according to any one of claims 1 to 5, theaqueous solution comprising: 6 to 38 parts by volume of a 20 to 25 wt %aqueous chromic acid solution; 2 to 8 parts by volume of about a 35 wt %aqueous peroxide solution; and 100 parts by volume is 55°-35° Baumesulfuric acid.
 7. The method according to any one of claims 1 to 6,wherein the peroxide is hydrogen peroxide or barium peroxide.
 8. Themethod according to any one of claims 1 to 7, wherein the chromic acidis H₂CrO₄.
 9. The method according to any one of claims 1 to 8, whereinthe current is 20 to 80 mA/cm² (20 to 80 A/ft²).
 10. The methodaccording to any one of claims I to 9, the aqueous solution furthercomprising a catalyst.
 11. The method of claim 10, wherein the catalystis hydrogen peroxide.
 12. The method according to any one of claims 1 to11, wherein the aqueous solution is maintained at a temperature of 30 to80° C. (90° F. to 180° F.).
 13. The method of claim 12, wherein thesolution is maintained at 50 to 60° C. (120° F. to 140° F.).
 14. Themethod according to any one of claims 1 to 12, the cathode comprising ananodically deposited coating of lead, carbon or titanium or oxidesthereof, covering a conductive material having a backing ofnon-conductive material.